About Skew-T Log-P Charts

Overview

A skew-t log-p chart plots the temperature, dew point and winds above a specific point on the ground. For example, the following skew-t log-p chart illustrates the atmosphere above Oshkosh from ground level up to 45,000 feet on the y-axis. The x-axis includes the temperature which is skewed to the right at a 45-degree angle. The thin red line that travels upward to the right from the 0 degree tick up to a pressure altitude of about 27,000 feet is called an isotherm. The thick red line plots the temperature at various altitudes. The thick blue line plots the dew point at various altitudes. In this example the dew point and temperature are converging between 10,000 and 20,000 feet which indicates saturation of the air and thus, clouds. Since these clouds are to the left of the 0-degree isotherm they are below freezing which means icing is a possibility. This particular chart would tell the non-FIKI pilot to stay below 10,000 feet in this area.

Winds aloft are depicted on the right side of the chart. The red barbs indicate the direction and velocity of the wind at a specific pressure altitude. In this example the winds are shifting from the southeast to the southwest at around 10,000 feet. To the right of the barbs is a thin blue line depicting the wind velocity in knots.

The skew-t log-p chart was invented in 1947 as a tool for the United States Air Force to evaluate the atmosphere. According to Wikipedia:

“In 1947, N. Herlofson proposed a modification to the emagram which allows straight, horizontal isobars, and provides for a large angle between isotherms and dry adiabats, similar to that in the tephigram. It was thus more suitable for some of the newer analysis techniques being invented by the United States Air Force.”